Known multi-layer printed circuit boards of low dielectric constant include laminates composed of a fluoroplastic and a glass cloth reinforcing sheet. However, these circuit boards have low strength, and a fluoroplastic film serving for adhesion of laminates should be heated to a high temperature of 350.degree. C. or higher to exert its adhesion.
Thermosetting resin laminates using fluoroplastic fabric as a base material are also known, but they similarly have low strength. Also known are laminates comprising glass fabric as a base material and a thermosetting resin matrix having incorporated therein a fluoroplastic powder, but they are inferior in adhesion to a metal foil.
In addition, known laminating materials for a multi-layer printed circuit board include laminates having a combination of glass cloth/epoxy resin, a combination of glass cloth/fluoroplastic, a combination of fluoroplastic fabric/epoxy resin, and the like. In any of these materials, there is a large difference between the reinforcing base material and the resin in dielectric constant, and a change of resin content in the laminate is attended by a considerable change of dielectric constant. In multi-layer printed circuit boards, since copper-clad laminates or adhesive prepregs having various thicknesses are used, it has been difficult to regulate the resin content in each insulation layer constituting the multi-layer printed circuit board. Accordingly, the insulation layers show scatter of dielectric properties among the laminate. In particular, such scatter gives rise to a serious problem as the dielectric constant of the laminating material becomes lower.